Etching silicide coatings and article formed therefrom



United States Patent C "ice ETCHING SILICIDE COATINGS AND ARTICLE FORMED THEREFROM Bruce A. Fosnocht, Loveland, Ohio, assignor to General Electric Company, a corporation of New York No Drawing. Filed Nov. 28, 1967, Ser. No. 686,314 Int. Cl. B32b 15/04; C23f 1/00 US. Cl. 161207 7 Claims ABSTRACT OF THE DISCLOSURE Silicided molybdenum can be rendered more resistant to oxidation at high temperatures by subjecting the silicided molybdenum to the action of molten sodium hydroxide at elevated temperatures, whereby the silicided coating is reduced in thickness. The treated silicided molybdenum articles prepared by this invention find use in chemical reactors which are subjected to oxidation at high temperatures and in heating elements.

This invention relates to a method for forming an improved oxidation resistant coating on molybdenum which has been silicided. More particularly, this invention is concerned with a method of etching a silicided molybdenum article whereby the silicide coating is reduced in thickness and the oxidation resistance of the article is improved.

-It is known that uniform adherent, tough, corrosion resistant silicide coatings can be formed on molybdenum by immersing the molybdenum article in a fused salt bath composed essentially of at least one alkali metal fluoride and from 0.5 to 50 mole percent of at least one alkalimetal fluosilicate. The process which is described in Reissue Pat. 25,630 and which is included herein by reference, comprises subjecting the molybdenum article to a direct electric current employing a silicon anode and using the molybdenum article as a cathode. The silicon ions discharged at the surface of the molybdenum cathode immediately diffuse into and react with the molybdenum to form a silicide coating.

Although the silicide coatings obtained by the electrolytic fused salt bath method described in the above identified reissue patent produce a silicided molybdenum surface which is extremely resistant to oxidative degradation, it has been found that when these silicided molybdenum articles are heated to exceptionally high temperatures in the range from 1200 to 1600 C. or higher, the silicon continues to diffuse into the molybdenum, thereby rendering the surface less resistant to oxidation and the entire article more brittle and subject to failure.

It has been found that the undesirable migration of the silicon into the molybdenum article can be prevented and the silicided molybdenum article rendered more resistant to oxidation by subjecting the silicided molybdenum article to the action of molten sodium hydroxide at temperatures ranging from above the melting point of the NaOH, whereby a portion of the silicided molybdenum is removed by the action of the molten sodium hydroxide bath.

In accordance with the process of the present invention, a molybdenum article which has a silicided molybdenum surface having a depth of greater than 1 mil in thickness is placed in a bath of molten sodium hydroxide, whereby the sodium hydroxide reacts with the silicon and molybdenum, etching a portion of the silicided molybdenum from the surface and rendering the silicided molybdenum more resistant to oxidation at elevated temperatures.

In the process of the instant invention, the molybdenum surface is one which has been silicided so that the silicide coating is in excess of 1 mil thick and can be as much as 3,532,591 Patented Oct. 6, 1970 10 mils in thickness. The silicided molybdenum surface is treated in this manner so as to remove a portion of the silicide coating so that the surface has a silicide coating having a final thickness of from 1 to 3 mils. The silicided molybdenum employed in the process of this invention is produced in accordance with the process described in the above-identified reissue patent assigned to the same assignee as the instant invention.

The temperature at which the molten sodium hydroxide bath employed in the instant process is maintained is not narrowly critical and can range from above the melting point of the sodium hydroxide and preferably from 325 C. to 800 C. or higher. It is preferred to employ a molten sodium hydroxide bath in a temperature range of from about 450 C. to about 500 C., since at higher temperatures more careful control must be maintained to prevent too much of the silicided molybdenum surface from being removed.

The sodium hydroxide employed in the process of this invention can be reagent grade sodium hydroxide or technical grade sodium hydroxide and the like. The sodium hydroxide bath can also contain small amounts, up to 10%, by weight, of the normal impurities present in sodium hydroxide such as sodium oxide, sodium carbonate, etc., or small amounts of water. The sodium hydroxide bath can also contain up to 10%, by weight, of additives such as sodium nitrate or sodium nitrite which cause a more rapid etching of the silicided molybdenum surface; however, extreme care must be exercised to make certain that too much of the surface is not removed. However, it is preferred to employ sodium hydroxide alone in the process of this invention.

When lithium hydroxide, potassium hydroxide and cesium hydroxide are substituted for sodium hydroxide, in the process of this invention, although the coating is etched, no detectably significant improvement in oxidative stability is obtained.

The silicided surfaces employed in the process of this invention can be prepared in a number of ways; for instance, in accordance with the aforementioned reissue patent or they can be prepared by subjecting the molybdenum surface to vapors of chlorosilanes and decomposing the chlorosilanes thermally to yield the silicide coating on the molybdenum surface.

Although the mechanism whereby the process of this invention improves the oxidative resistance of the silicided molybdenum surfaces is not understood, it is believed that the sodium hydroxide in the bath penetrates the interstices of the silicided molybdenum surface and upon removing from the bath and washing and then being subjected to elevated temperatures there is residual sodium hydroxide in the interstices which reacts with the silicon and oxygen in the air to yield a sodium silicate glass which through Wetting and penetrating processes along large and microfine cracks in the coating, forms a sodium silicate or silica barrier preventing the migration of the silicon further into the molybdenum article thereby giving it greater oxidation resistance.

The silicided molybdenum articles produced in accordance with this invention find use in heating elements, combustion chambers, casting molds and chemical reactors, etc., which are subjected to elevated temperatures in oxidative atmospheres. The silicided molybdenum articles can be used to fabricate heating elements for use in air to prevent oxidative attack of the element at high temperature, to make turbine blades for both gas and steam driven turbines to resist the corrosive and erosive effects of the gaseous driving fluid.

The following examples serve to further illustrate this invention. The examples are given by Way of illustration and not by way of limitation.

3 EXAMPLE 1 An 11 inch length of 0.025" diameter centerless ground molybdenum rod was bent into a hairpin shape and cleaned anodically in sulfuric acid. The filament Was then wet vapor-blasted and scrubbed with cleanser prior to coating.

The silicide coating Was applied electrolytically in a molten fluoride electrolyte at a current density of 0.9 amp/dm. for 3.5 hours at 675 C. 37.1 mg. of silicon were deposited and the filament gained 1.8 mils in diameter.

After coating, the filament Was dipped into a bath consisting of molten NaOH, of reagent grade, at 475 C. three times until it had lost 18.1 mg, a Weight equal to almost half the silicon initially deposited. The wire was next scrubbed in cleanser, during which operation a black deposit was removed, and the filament lost 3.7 mg. more. In this stripping and scrubbing operation, the Wire lost 0.7 mil in diameter. This filament, which now had 1.45 mil thick silicided molybdenum coating, was resistance heated in a water-cooled electrode to 1500 C., which temperature was measured with an optical pyrometer, using an emissivity factor of 0.65.

The filament failed after 312 hours at 1500 C. Failure occurred at a local hot spot, and the remainder of the surface appeared smooth with very little apparent change. Samples of silicided molybdenum wires which had not been treated with sodium hydroxide and which had thicknesses of the silicide coating from 1 to 6 mils in thickness when subjected to internal electrical heating to 00 C., by passing an electric current through the wires had much shorter lifetimes which followed a straight line on a log-plot of coating thickness versus lifetime of the filament. For example, a molybdenum wire having an original diameter of 0.025" was silicided to give a 2 mil thick silicide coating on the filament; this filament had a lifetime of 12.8 hours at 1500 C. A similar filament which had a silicide coating thickness of 6 mils had a lifetime of 14.7 hours at 1500 C.

EXAMPLE 2 A molybdenum filament having an original diameter of 0.025 inch was silicided as in Example 1 by the procedure of Reissue Pat. 25,630 to give a 2.65 mil thick silicide coating on the filament. The filament was then subjected to the molten sodium hydroxide bath at 475 C. to yield a filament having a 2.15 mil thick silicide coating. The filament was washed as in Example 1, dried and heated electrically to 1500 C. by passing an electric current through the filament. The filament had a lifetime of over 2500 hrs. at which time the test was discontinued.

4 EXAMPLE 3 A number of molybdenum filaments (0.025 inch diameter) were silicided as the filaments of Examples 1 and 2 and subjected to the molten sodium hydroxide bath as in Examples 1 and 2 and then tested by heating the filaments to 1500 C. electrically by passing a current through the filaments to give the results shown in the following table.

TABLE I Thickness Number original times Final Life time at coating, dipped at thickness, 1,500 C., mils 475 C. mils hours 1 Heated to 1,650 C. for additional 63 hours before failure.

It will, of course, be apparent to those skilled in the art that modifications other than those set forth in the above examples can be employed in the process of this invention without departing from the scope thereof.

What is claimed is:

1. A process for improving the oxidative resistance of a silicided molybdenum surface which comprises subjecting said surface to a molten bath composed essentially of sodium hydroxide, thereby removing a portion of the silicided molybdenum surface.

2. A process as in claim 1 wherein the silicided molybdenum surface comprises a silicided molybdenum wire.

3. A process as in claim 1 wherein the thickness of the silicided molybdenum surface after treatment is from 1 to 3 mils.

4. A process as in claim 1 wherein the temperature of the sodium hydroxide is between 325 to 800 C.

5. A silicided molybdenum surface produced in accordance with the process of claim 1.

6. A silicided molybdenum wire produced in accordance with the process of claim 2.

7. A process as in claim 1 wherein the temperature of the sodium hydroxide is between 450 to 500 C.

References Cited UNITED STATES PATENTS 3,086,886 4/1963 Kietfer et a1. 117-l02 ROBERT F. BURNETT, Primary Examiner W. A. POWELL, Assistant Examiner US. Cl. X.R. 

